1. Field of the Invention
The present invention relates to an ink jet printing apparatus. In particular, the present invention relates to the control of a timing at which ink is ejected through a printing head in synchronization with an operating for conveying a print medium.
2. Description of the Related Art
In recent years, digital copiers and printers have been rapidly diffused. Since digital printing system are effective for color adjustment or image processing for example, they have been increasingly used in the field of a color printing apparatus such as a color printer or a color copier. On the other hand, printing apparatuses can be classified to the electronograph one, the ink jet one, or the thermal transfer one for example among which the ink jet printing apparatus is advantageous in that three factors of the cost of the apparatus, the printing quality, and the running cost. Thus, digital color ink jet printing apparatus have been useful in recent years in a range from a low-cost and small apparatus such as a household printer to a large apparatus such as the one for office use.
By the way, more digital cameras have been recently used with a diffusion rate higher than that of silver salt photograph cameras. Thus, large-scale retailers (labo), which conventionally have provided a service for developing silver salt photographs and a print service, recently provide a digital print service for images taken by digital cameras. Such a labo is required a large amount of print output within a short time. Thus, the labo frequently uses an ink jet printing apparatus that continuously conveys a continuous form paper (a print medium wound in a roll-like shape) to eject ink from a long printing head corresponding to the width of the print medium to print an image. The roll paper (continuous form paper) requires a lower cost than that for a cut paper because the manufacture does not require a cut processing and the roll paper can be fed into the apparatus by a simpler mechanism than that for a cut paper. This makes it possible to provide a printed matter with a relatively low cost while reducing the cost for the apparatus itself and the failure frequency. Furthermore, a combination of the use of a long printing head corresponding to the width of a print medium with the continuous feeding of a roll paper can provide a higher printing speed.
FIG. 6 illustrates the outline of a printing apparatus for using a long printing head (hereinafter simply referred to as a printing head) to print an image on a roll paper. A roll paper 6 wound around a rolling body (roll paper rolling body) 5 is disengaged from the rolling body 5 in accordance with the rotation of the rolling body 5 to enter a nip section between a resist roller 7 and an upper resist roller 8. The resist roller 7 and the upper resist roller 8 are rotated while the roll paper 6 being nipped between the upper and lower faces to convey the roll paper 6 to a printing section while correcting the inclination of the roll paper 6.
The downstream side of the resist roller 7 constitutes a printing section in which printing heads 1 to 4 for ejecting ink droplets for printing are arranged to be parallel with one another as shown in the drawing. The printing head 1 ejects cyan ink, the printing head 2 ejects magenta ink, the printing head 3 ejects yellow ink, and the printing head 4 ejects black ink. The respective printing heads 1 to 4 include a plurality of nozzles for ejecting ink that are provided in an amount corresponding to the width of the roll paper 6 in a direction crossing the conveyance direction. At a timing at which the roll paper 6 passes beneath the individual printing heads, ink is ejected from the nozzles of the printing head to form a full color image in a stepwise manner.
The convey path of the printing section includes five spur driving rollers 21 to 25 and five spurs 31 to 35 opposing to the spur driving rollers 21 to 25 as shown in the drawing. These five pairs of rollers function to maintain regions of the roll paper 6 subjected to printing operations by the respective four printing heads 1 to 4 in a flat manner. At the lower side of the regions at which the printing operations by the printing heads 1 to 4 are performed, platens 41 to 44 are provided to maintain distance between a printing surface and the nozzle surfaces of the printing heads while suppressing the roll paper 6 from moving in the downward direction.
At the further downstream of the spur 35, there are a paper ejection roller 9 and an upper paper ejection roller 10 that rotates to follow this paper ejection roller 9 to convey the roll paper 6 to a subsequent step (not shown) such as a cutter.
A speed for conveying the roll paper 6 as described above can be obtained by providing a rotary encoder for detecting the rotation speed of the resist roller 7 for example. In accordance with an output from this encoder, timings at which ink is ejected from the printing heads 1 to 4 can be adjusted to print dots on accurate positions on a roll paper.
FIG. 7 is a schematic diagram specifically describing the structure for adjusting the ejecting timing. In FIG. 7, the resist roller 7, the upper resist roller 8, and the printing head 1 are shown when seen from the conveyance direction of a roll paper. The center axis of the resist roller 7 is fixed to the center of a roller gear 803. The roller gear 803 is connected to a paper feed motor 801 via a driving transmission belt 802. Specifically, the driving force of the paper feed motor 801 is transmitted through the driving transmission belt 802 to rotate the roller gear 803 to further rotate the resist roller 7.
On a tip end of the center axis of the resist roller 7 a rotary encoder 810 is attached. The encoder 810 includes an encoder wheel 811 that is connected to the center axis of the resist roller 7 to rotate together with the resist roller 7 and two encoder sensors Ach 812 and Bch 813 that detect the scale of the encoder wheel 811 from both sides of the center axis.
When the driving force of the paper feed motor 801 is used to rotate the resist roller 7 in a printing operation, the two encoder sensors 812 and 813 output pulse signals TA and TB in synchronization with the scale of the encoder wheel 811 detected by the encoder sensors 812 and 813. If the resist roller 7 and the encoder wheel 811 are assembled with no error at all, the two pulse signals TA and TB are outputted in complete synchronization. However, in an actual case, a small error is always caused in the engagement between the resist roller 7 and the encoder wheel 811 and a position at which the encoder sensor is attached to the encoder wheel 811, thus frequently preventing TA and TB from being in complete synchronization. Consequently, a correction circuit 804 is generally provided that averages the cycles of the two pulse signals TA and TB based on (TA+TB)/2 to obtain an average cycle for generating a new pulse.
A pulse signal outputted from the correction circuit 804 is inputted to an ejecting control circuit 805. Based on the resultant pulse cycle, the ejecting control circuit appropriately controls the ejecting timings of the printing heads 1 to 4 in accordance with the positions of the printing heads 1 to 4.
FIG. 8 is a block diagram illustrating a method by a conventional ejecting control circuit 805 for controlling the ejecting timings of the printing heads 1 to 4. A printing start signal inputted from an input terminal 909 is inputted to the first ejecting timing generator 921 for generating an ejecting timing for the printing head 1. A corrected pulse signal outputted from the correction circuit 804 is also inputted to the first ejecting timing generator 921. Based on the printing start signal inputted from the input terminal, the first ejecting timing generator 921 generates a timing at which the printing head 1 ejects ink while being in synchronization with the pulse signal inputted from the correction circuit 804.
The printing start signal inputted from the input terminal 909 is also inputted to the first delay generator 902. The first delay generator 902 delays the printing start signal in accordance with a distance between the printing head 1 and the printing head 2 and the pulse signal inputted from the correction circuit 804 to output the delayed printing start signal to the second ejecting timing section 922. Based on the printing start signal outputted from the first delay generator 902, the second ejecting timing generator 921 generates a timing signal at which the printing head 2 ejects ink while being in synchronization with the pulse signal outputted from the correction circuit 804. Thereafter, ejecting timing signals for the printing head 3 and the printing head 4 are similarly generated.
By the series of operations as described above, an accurate control of a printing position can be achieved without having an influence by an error related to the conveyance system such as the paper feed motor 801, the roller gear 803, and the driving transmission belt 802.
However, the above structure allows ink to be ejected while in synchronization with a signal of the encoder provided on the axis of the resist roller. Thus, this structure cannot solve a conveyance error due to the eccentricity of the resist roller itself. Furthermore, when a conveyance belt is used to convey the roll paper, an uneven thickness of the conveyance belt also causes variation in the printing position. This problem also cannot be solved by the above structure.
The problem as described above can be solved to a certain level by using the structures disclosed, for example, in Japanese Patent Laid-Open No. 2006-192807 and Japanese Patent Laid-Open No. H04-226379. Japanese Patent Laid-Open No. 2006-192807 discloses a technique to detect an eccentric component in a print medium conveyance system to correct a printing position in accordance with the detected eccentric component. Japanese Patent Laid-Open No. H04-226379 discloses a technique to use a laser Doppler speedometer or the like to detect the conveyance speed of the conveyance belt so that ink can be ejected from a printing head while in synchronization with the resultant conveyance speed.
However, the structure as described with reference to the drawings and the structures as disclosed in Japanese Patent Laid-Open No. 2006-192807 and Japanese Patent Laid-Open No. H04-226379 can correct the error owned by a target mechanism itself but do not directly detect the conveyance status of an actually conveyed print medium. Thus, it has been impossible to suppress a dislocated printing position caused when a roll paper deflects among a plurality of rollers or meanders in conveying or when slippage is caused between a print medium and a roller.
FIGS. 9A and 9B are a schematic diagram illustrating a dislocated printing position caused when the roll paper (print medium) 6 deflects between two pairs of rollers. FIG. 9A shows a status where no deflection is caused. FIG. 9B shows a status where deflection is caused.
When there is no deflection between the two pairs of rollers as shown in FIG. 9A, the roll paper retained between the printing head 1 and the printing head 2 has a length d1 equal to a distance D between two printing heads. However, when deflection is caused between the two pairs of rollers as shown in FIG. 9B, the roll paper retained between the printing head 1 and the printing head 2 has a length d2 that is longer than the distance D between the two printing heads. In this case, a longer time is required for a predetermined position in the roll paper 6 to pass just below the printing head 1 to arrive at a position just below the printing head 2 than in the case where there is no deflection. However, since the conventional structure does not directly detect the conveyance amount of the print medium, the conventional structure does not consider this delayed arrival. As a result, even data for an identical raster position is printed at different positions on a print medium by the printing head 1 and the printing head 2. Specifically, dislocated position is caused on the print medium in the conveyance direction. Thus, dislocated color is caused when different colors are used by the printing head 1 and the printing head 2.
Generally, a roll paper is stored, just before a printing operation, while the printing surface being wound. Thus, a roll paper cannot prevent some winding pattern and thus tends to cause the deflection as described above. However, the conventional method could not directly detect the convey status of an actually conveyed print medium and thus could not avoid an adverse effect due to the deformation of a print medium itself such as the deflection. In addition, the deformation of a print medium is not limited to the roll paper and is caused also by using a cut paper.